C# 14: User defined compound assignment operators.

[michaelnebel]

In this PR we introduce support for user-defined compound assignment operators.

class A {
    // Compound operator declaration.
    public void operator +=(A x) {
       ...
    }
}

class B {
    public void M(A a1, A a2) {
        a1 += a2; // Compound operator invocation.
    }
}

Compound assignment operator calls are not implicit assignments, but instead the left hand side of the compound assignment is updated. That is, user-defined compound operator calls behave more like instance method calls than static operator calls.
Had we instead declared

class A {
    public static A operator +(A x1, A x2) {
        ...
    }
}

then a1 += a2 is also valid, but instead it means a1 + a2 and assign the result to a1.

Notes on the implementation

• Even though it is claimed that there is some support for dynamic operator calls, I could not produce a running example (and thus haven’t introduced dispatch logic for this case).
• There will be a separate PR for the increment/decrement operator.

[michaelnebel]

DCA looks good

• Performance doesn't appear to be affected.
• Fewer extraction errors.

[Copilot]

Pull request overview

This PR adds C# 14 support for user-defined compound assignment operators (for example a += b where operator += is declared) by extracting them as operator invocations rather than desugaring them into a = a + b, and updates QL dispatch/dataflow plus tests accordingly.

Changes:

• Add new extractor handling for compound assignment expressions to emit direct operator-call extraction for user-defined (instance) compound assignment operators.
• Extend the C# QL library to represent and dispatch CompoundAssignmentOperatorCall distinctly from ordinary OperatorCall.
• Update/extend AST and dataflow library tests and expected outputs for compound assignment operator declarations and invocations (including extension-based operators).

Reviewed changes

Copilot reviewed 19 out of 20 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
csharp/ql/test/library-tests/operators/operators.cs	Adds user-defined compound assignment operator declarations and invocations for AST coverage.
csharp/ql/test/library-tests/operators/PrintAst.expected	Updates AST expectations to include compound assignment operators/calls.
csharp/ql/test/library-tests/operators/Operators1.expected	Updates operator location expectations after AST shifts.
csharp/ql/test/library-tests/operators/Operators2.expected	Updates operator location expectations after AST shifts.
csharp/ql/test/library-tests/operators/Operators3.expected	Updates conversion-operator location expectations after AST shifts.
csharp/ql/test/library-tests/operators/Operators4.expected	Updates conversion-operator location expectations after AST shifts.
csharp/ql/test/library-tests/dataflow/operators/Operator.cs	Adds a dataflow test for user-defined compound assignment operator calls.
csharp/ql/test/library-tests/dataflow/operators/operatorFlow.expected	Updates expected dataflow graph to reflect compound assignment operator modeling.
csharp/ql/test/library-tests/dataflow/extensions/extensions.cs	Adds an extension-based compound assignment operator and a flow test covering it.
csharp/ql/test/library-tests/dataflow/extensions/ExtensionFlow.expected	Updates expected dataflow output for the new extension compound assignment test.
csharp/ql/lib/semmle/code/csharp/exprs/Call.qll	Introduces CompoundAssignmentOperatorCall in the QL expression hierarchy.
csharp/ql/lib/semmle/code/csharp/dispatch/Dispatch.qll	Adjusts dispatch to treat compound assignment operator calls as dispatchable calls with a qualifier.
csharp/ql/lib/semmle/code/csharp/Callable.qll	Adds QL operator classes for compound assignment operator declarations.
csharp/ql/lib/change-notes/2026-03-06-compound-assignment-operations.md	Adds changelog entry for the new C# 14 support.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/UserCompoundAssignmentInvocation.cs	New extractor expression node for user-defined compound assignment operator invocations.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Invocation.cs	Refactors call target resolution to use a shared helper.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Factory.cs	Routes compound assignment syntax kinds through new compound-assignment extraction logic.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/CompoundAssignment.cs	New helper to decide between desugaring vs direct operator-call extraction for compound assignments.
csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs	Adds ExpressionNodeInfo.GetTargetSymbol helper for consistent target resolution.
csharp/extractor/Semmle.Extraction.CSharp.Util/SymbolExtensions.cs	Extends operator-symbol parsing to recognize checked and assignment operator method names.

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[hvitved]

LGTM, some trivial comments.

[hvitved]

Nit: Can be a ReadOnlyDictionary.

[hvitved]

missing comma

[hvitved]

Alternatively i >=0 and if this.isOrdinaryStaticCall() then j = i else j = i - 1

[michaelnebel]

Putting this back in Draft as there has been significant changes in main (removal of expanded assignments).